In many Micro-Electromechanical System or MEMS devices, electrostatic actuation is used to move micromechanical structures. For example, one type of MEMS device that uses electrostatic actuation is a ribbon-type spatial light modulator, such as a Grating Light Valve (GLV™) commercially available from Silicon Light Machines, Inc., of Sunnyvale, Calif. Referring to FIGS. 1A and 1B, a ribbon-type spatial light modulator 100 generally includes a number of ribbons 102a, 102b; each having a light reflective surface 104 supported over a surface 106 of a substrate 108. One or more of the ribbons 102a are deflectable through a gap or cavity 110 toward the substrate 108 to form an addressable diffraction grating with adjustable diffraction strength. The ribbons are 102a deflected towards the surface 106 of the substrate 108 by electrostatic forces when a voltage is applied between electrodes 112 in the deflectable ribbons 102a and base electrodes 114 formed in or on the substrate. The applied voltages are controlled by drive electronics (not shown in these figures), which may be integrally formed in or on the surface 106 of the substrate 108 below or adjacent to the ribbons 102. Light reflected from the movable ribbons 102a adds as vectors of magnitude and phase with that reflected from stationary ribbons 102b or a reflective portion of the surface 106 beneath the ribbons, thereby modulating light reflected from the SLM 100.
One chronic problem encountered with conventional electrostatically operated or voltage controlled MEMS devices is referred to as “snapdown.” More specifically, when the voltage applied to an actuating electrode 112 in such device exceeds a critical value, roughly that required to deflect the membrane or movable ribbons 102a beyond one third of the initial gap 110, the attractive force between surfaces can exceed a linear restoring force of the membrane resulting in an unstable pull-in of the surfaces also called “snapdown”. Moreover, atomic-level bonding forces frequently exceed the restoring force of the membrane structure, causing the membrane to remain “stuck” to the surface of the substrate permanently damaging the ribbon and rendering the MEMS device inoperable.
Accordingly, there is a need for a circuit and method that reduces or substantially eliminates snapdown in voltage controlled MEMS devices.
The present invention provides a solution to these and other problems, and offers further advantages over conventional MEMS devices and methods of operating the same.